Need help with my prop

I have a 1972 18 ft aluminum deep v starcraft boat with a 2011 60 horse Mercury four stroke on it. I bought it a couple years ago and I would like some information on changing the prop. I have no idea about prop sizes and prop pitches and any information would help. The prop is a stainless steel prop but it seems very small to me. I measured it across and it's a tad under 10 in if I am measuring correctly. With me and one other person in the boat I can get it up to about 36 mph but it takes a while to get there. If I put four people in that boat it will hardly plane out and I can only get 16 to 18 mph. 36 mph is great but I would rather have more low end and be able to haul more people. Also I have a very tough time pulling a tube. I am wondering if I could go with a bigger prop or a pitch change to get more low end out of it. I don't know how to tell what pitch my prop is. I appreciate any and all suggestions thank you

If you're measuring around 10", then you don't have a BigFoot, and that motor probably came with a 10-3/8 x 13" or 14" 3-blade.

If you want to carry more people, stuff, and pull tubes, you may want to consider a 4-blade over a 3-blade simply to get on plane easier and maintain it under load.

I'll let the experts here chime in on size recommendations. I was thinking a 10.3 x 13 4-blade SpitFire but eh... there's a lot to consider

Is there foam under the floor?---Possibly soaked with water?----What is the pitch on the prop that you have now ?

What's the max HP rating for the boat? You may be under powered too.

Definitely under powered.
I had the same boat model and year.
65 hp was not enough, as you discovered.
You need a more hp
I believe it is rated for 125 hp.
Put a 90, 115, or 125 on it and enjoy boating.

Agree with the above, 85+ hp minimum for 4 people in a 18 SC aluminum pulling tubes and all. My 18 Starchief topped out at 32 on a lucky day with an 85 and 3 people. My significantly lighter 16 ft SC supersport was good with 70 but also you could tell wanted its rated 85 with more than 2 or 3 people plus things like coolers and all that. Both of those are the standard shallower design hull than your deep-V so typically you'd need a bit more power for same results. Also those were lighter 70's and 80's 2 stroke motors. Prob similar weight to a modern 4 stroke 60.

Higher power to weight has some hidden advantages.
Reserve power is great for emergencies but it also means you're usually not running the engine nearly as hard. Generally you can get way better actual efficiency per gallon from your motor running it in the 70% throttle or less range. Also longer life, less maintenance issues, risk of blowing it up, no special props needed, etc. Counterintuitively- sometimes a bigger motor is significantly more efficient over all.